Astronomy Terms and Definitions

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Astronomical Unit (AU)

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The (average) distance between the Earth and the Sun

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Light-Year (ly)

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The distance light travels in one year (9.46 \times 10^{12} km or 9,460,000,000,000 km)

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Parallax

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A technique used to determine the distance of nearby stars by observing their "shift" in position based on background stars

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Apparent Magnitude

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The brightness of a celestial object as seen from Earth

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Absolute Magnitude

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The inherent brightness of an object as if it were a standard distance of 10 parsecs from Earth

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Spectroscopy

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Used to determine the chemical composition of stars

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Sidereal Day

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The time it takes Earth to complete one rotation relative to distant stars; shorter than a solar day (23 h 56 min)

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Parsec (pc)

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Equal to 3.26 ly (3.086 \times 10^{13} km or 13,103,086,000,000,000 km)

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Cosmic Calendar

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A method to reasonably visualize and comprehend the chronology of the universe's 14 billion year history

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Kiloparsec (kpc)

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1,000 parsecs